The present invention is an improvement to welding power supplies particularly suited for environments in which a relatively large number of welders are working in the area. The present invention is particularly useful in environments in which welding must be done in relatively small cramped areas. In particular, the environment of a shipyard is one in which the present invention is particularly useful.
As is known to those skilled in the art, welding jobs in shipyards normally include a relatively large number of welders welding simultaneously. Additionally, in the construction or repair of a ship, it is often necessary for a welder to work in relatively cramped quarters within the interior of a ship.
Prior art arrangements have normally included a relatively large number of power supplies, one for each welder. Such an arrangement is time consuming and expensive since it requires multiple drops of three-phase high voltage AC lines to supply the various power supplies. As is known to those skilled in the art, accidental cutting of high voltage AC power lines leads to much more dangerous situations than similar accidents involving the lower voltage DC lines.
Furthermore, lack of reasonable access to the AC power lines and/or the impracticality of or undesirability of moving the prior art power supply sometimes creates a situation wherein the prior art power supply is substantially removed from the welder and a long rum of welding cable is required. Since the low voltage high current outputs of such welding power supplies require large gauge cables, pulling long runs of these cables around a shipyard or similar environment is cumbersome, results in increased I.sup.2 R losses as the cable length increases, and causes variations in the quality of the weld because of the distributed inductance, capacitance and resistance of long runs of cable.
Additionally, the use of multiple individual welding power supplies necessarily means duplication of one of the most expensive components of a power supply, a large, high current three-phase transformer.
Thus, as explained in greater detail hereinbelow, the present invention provides a distributed welding power supply system which overcomes some of these disadvantages. The system of the present invention generally consists of a single high power regulated power supply and a plurality of distributed weld selector stations. Within each weld selector station there are a number of elements essentially similar to a smaller welding power supply. Therefore, it is appropriate to consider other aspects of the background of the art of solid state welding power supplies in connection with the weld selector stations used in the present system.
Again considering the environment of a shipyard as a typical environment for welding, it is highly desirable to provide a power supply which is usable in a relatively large number of welding processes. In particular, there are a number of variations in the electrical output characteristics of a welding power supply which affect its utility in particular welding processes. Among these are the output impedance and the turn-on and turn-off time or the dV/dt (dI/dt) characteristics of the voltage output (current output) of the supply. For example, in tungsten inert gas (TIG) and stick welding, it is generally desirable to have a power supply with a high output impedance so that its characteristics approximate a constant current source. In metal inert gas (MIG) and pulse arc welding, it is often desirable to have a power supply with a low output impedance which approximates a constant voltage source. There are, of course, other types of welding which require a compromise between these characteristics.
Another aspect of a welding environment well known to those skilled in the art is the fact that large transients are present in the main current carrying cables within a welding system. The use of pulse width modulators to control solid state switching devices to adjust output voltage has been known in the art for some period of time. Generally, such pulse width modulators are constructed using relatively low voltage solid state integrated circuits. Such circuits are provided with well regulated and well by-passed low voltage power supplies so that they will operate properly.
One problem with prior art power supplies of this type occurs from transients which occur on the DC input to a welding power supply switching regulator operated by the pulse switch modulator. When an arc is struck, there is normally a large current surge initially drawn from the output of the power supply which normally lowers the voltage at the input of the regulator. A problem which arose in designing a system of the present invention was the fact that multiple welders are operating weld selector stations off of a single power supply output. As various welders using the system strike arcs, the overall output voltage of the main supply, and thus the input voltage to the various weld selector stations, will drop. Thus, it is desirable to provide a feedback system for pulse width modulators used in a distributed welding power supply system which will respond to changes in the input voltage to the weld selector station.
Additionally, in some welding environments there are a large number of remote welding units which include wire feed motors and shielding gas valves, for example, of the type shown in applicant's U.S. Pat. No. 4,119,830, issued Oct. 10, 1978. The voltage supply to the wire feed motor normally fluctuates with the voltage supplied by the power supply output. Also, in some types of MIG welding, the output voltage is lower than that required to drive the wire feed motor. Thus, there is a need to assure that the voltage to the wire feed motor will not become excessive so as to damage the motor or inadequate so as to cause stalling or erratic operation.
Furthermore, in many prior art systems, a high power resistance box is used to obtain the desired output voltage, current and impedance. These boxes can consume substantial amounts of power which must be dissipated as waste heat, lowers the energy and cost efficiency of the system, and requires additional air flow or cooling.
Lastly, there is a problem in the state of the prior art, not limited to the environment of welding power supplies, which has been overcome by the present invention. As is well known to those skilled in he art, the design of reliable push-pull amplifiers, whether they be linear amplifiers or push-pull devices used in switching power supply regulators, have always been considered to require closely matched pairs of output devices. This goes back to the days of push-pull vacuum tube audio amplifiers. In the design of push-pull solid state amplifiers, this problem has become more critical since excessive gain in one transistor of a push-pull amplifier stage normally leads to greater power dissipation and thus a higher operating temperature for that particular transistor. As the operating temperature increases, the beta of the transistor tends to increase, and this ultimately leads to a condition known as thermal run-away in which the higher beta transistor will be destroyed.
As is also known to those skilled in the art, matched pairs of transistors of a particular type tend to be considerably more expensive than individual transistors of the same type. This cost increase becomes even greater when relatively high current devices, such as those used in relatively large switching power supplies, are used. Thus, there is a need in the art to provide a practical and reliable arrangement for designing push-pull amplifier output stages (which may be used in switching power supplies, linear amplifiers, and other applications for push-pull topology) which can eliminate the requirement for transistors having closely matched betas and thermal characteristics.